A method and apparatus for three-dimensional image-rendering of a spatial and tissue-based configuration through separating high contrast and injected contrast agents in multi-angular X-ray absorption measurement.
The invention relates to a system as recited in the preamble of Claim 1. The method applies to a tissue-based configuration, which tissue actually represents or originates from living matter. Usually, although not restrictively, the various angles stem from a rotational motion only of the direction of radiating. Known volume reconstruction in 3D-RX has for some time used the algorithm as first published in L. A. Feldkamp et al, Practical Cone Beam Algorithms, J. Opt. Soc. Am. A6 (1984) pp. 612-619. Certain results of the procedure have been published in J. Moret et al, 3D rotational angiography: Clinical value in endovascular treatment, Medica Mundi Vol. 43, Issue 3 (November 1998), pp. 8-14, and in R. Kemkers et al, 3D-Rotational Angiography: First clinical application with use of a standard Philips C-arm system, Proc. CAR ""98, pp. 182-187. In these two publications, a high spatial resolution combines with large contrast differences as distinguished from Computer Tomography. The high-pass filtering used causes a good discrimination of high contrast objects, thereby allowing the use of non-subtracted rotational images. Reconstructed images show good visibility of items such as arteries and aneurysms that are filled with a contrast-enhancing substance. Through volume-rendering techniques, visualization can assign the available grey-levels to calculated high-contrast structures in an optimum manner. The method may find application in various branches of medicine, such as neural surgery and osteopathology.
However, the existing presence, or the interventional providing of externally provided objects for remedial and other purposes, such as platinum coils, clips, and stents (hereinafter xe2x80x9cobjectsxe2x80x9d), will introduce volumes of a much higher contrast-density than the highest density associated to contrast fillings. This means a reduced selectivity when visualizing the latter items, sometimes to the effect of sub-standard definition of the transitions between interventional contrast fillings and the above discrete objects. Another curative intervention in the tissue can be the introducing of a self-fixating glue for filling aneurysms and arterio-vascular malformations or AVMs. Here, the introduced matter would not impede the voluminal rendering, but rather could the assessing of an adequate filling or otherwise of the intended vessel with glue become more or less invisible in the process of visualization.
Conventional rotational angiography has been equipped with a subtraction feature between two successive runs: one run a mask image without contrast, and the other run a contrast image, both of them being 2D-images. Visualizing has been effected through on-the-fly subtracting of the two runs, to immediately produce the introduced contrast difference for the projection run. This immediate subtracting may however obscure many pre-existent feature structures such as bone and xe2x80x9coldxe2x80x9d objects. By itself, accurate subtraction proved attainable through a newly attained high degree of spatial reproducibility and temporal stability of the apparatus. This has obviated the need for repeated calibration that need no longer be executed after a small number of images already. Finally, the immediate subtraction of the two runs of data means that the xe2x80x9cbackgroundxe2x80x9d image is no longer in use.
The inventors have recognized that the immediate subtraction of the two images has various disadvantages, and have recognized the feasibility of reversing the sequence of operations in that first a 3D image would be calculated, which image is subsequently compensated for unwanted features that thereupon will be excluded from the rendering process. The amended procedure has proven to allow more and easier manipulating of images with respect to size and scale, viewing angle, discrimination between various elements featured in a configuration, such as the elements of the tissue, the glue, and implant objects. Moreover, the subtraction of the 3D images to a certain extent smears out small errors that may be due to spatial discrepancies.
From this moment on, various possibilities are open: A first feature is that a 3D image may be based on the high contrast matter or HCM only without complicating vessel structures, such as for assessing correct object manipulation.
In the second place, the high-contrast material or HCM voxels may get a low greyness value, so that only the vessels get rendered in a region of interest with widely varying greyness values.
The two preceding results may be combined through temporally non-uniform procedures, such as by blinking, alternating, or by rendering in specific colours and/or greyness profiles.
The HCM voxels may be presented as a separate surface-rendered and colored volume.
In consequence, amongst other things, it is an object of the present invention to provide an improved imaging technology through separating the HCM image from the low absorption image for allowing improved manipulation of various parts of the image. Furthermore, the rendering may allow to include both transient and also temporally steady features. Now therefore, according to one of its aspects the invention is characterized according to the characterizing part of Claim 1.
Additional advantages of the present invention would be as follows. In using the prior art immediate subtraction, each pixel area enters the subtracting operation separately. Thereby, even a small shift between a mask image and the corresponding contrast image can cause spatial inaccuracies in transitions, both between respective arteries and the like, and also between HCM and non-HCM elements. In particular, bone structures can get enhanced and cause perturbance. The improved procedure of the present invention may now cause such inaccuracies exclusively in HCM-based transitions. The volume-determination would spread out certain inaccuracies in space, as it were.
Furthermore, due to variations in the image intensification factor, such as under influence of a non-steady high voltage of the radiation detection chain, and due to the absence of a reference image on the basis of a single contrast image, the image information according to the present invention may be non-linear and less than ideal. In the final subtracted image, this may sometimes cause non-linear errors, such as beam-hardening, saturation, etcetera, in places where HCM-object contrast and tissue-based contrast will overlap. Through the used high-pass reconstruction filter, this may cause further objects in the 3D reconstructed volume. However, in contrast with prior art, the novel procedure of subtracting after generating the 3D image, will not further amplify these adverse effects. The presence of HCM may justify applying the new method, in view of the increased X-Ray dose for the patient that is caused by the second run. In practice, the second run proves to provide a richer image content, as compared to the usage of only a single run. Note that the manipulating of the image as used herein is understood to mean the influencing of the image representation, without the apparatus knowing what the various pixels effectively mean.
The invention also relates to an apparatus being arranged for executing a method as claimed in Claim 1. Further advantageous aspects of the invention are recited in dependent Claims.